The Medieval WarmPeriod (MWP) refers to a time interval between AD 900 and 1300 in which some Northern Hemisphere regions were warmer than during the period known as the Little Ice Age that followed it, and also warmer than the period of glacial advance preceding the MWP. There is clear evidence in parts of the Northern Hemisphere for dating the Medieval Warming Period, both through glacial geologic dating and through recorded human exploration and history; however, there is some debate over the geographic comprehensiveness of the MWP; for example the presence of this period of warming on the Colorado Plateau is not clearly defined.[1]

Large extents of Iceland were farmed in the 10th century AD. At this time, Norse Vikings colonised Greenland, while a reduction of sea ice allowed regular voyages at these northern latitudes. There was a certain spatial inhomogeneity regarding the precise onset of the MWP, based upon different positions around the globe as Jansen et al have pointed out; the warmest conditions occurred at slightly different times in different locations: temperature peaked between 950 and 1200 in European Russia and Greenland, but somewhat later, between 1150 and 1300 in the majority of Europe

Glacial geologic dating

Key evidence for asserting the date range of the Medieval Warm Period is the analysis of residual moraines pushed by the most recent glacial advances.[2] The hypothesis for dating the MWP rests on the assumptions that the dates of the moraines preceding and following the interim warm period are reasonable approximations to the date termini of the MWP. More specifically, dating of organic material above a moraine yields a minimal age for that moraine's formation, while the material underlying the moraine provides an upper bound for its age.

Tree ring dating

Analysis of tree rings from ancient forests, also called dendrochronology, has yielded similar dates for the Medieval Warm Period.

For example, in section 14.6 of the Arctic Climate Impact Assessment (ACIA) a detailed analysis of well preserved ancient trees indicated a clear definition of the MWP as circumscribed by the 10th through 13th centuries AD; the ACIA is an international project of the Arctic Council and the International Arctic Science Committee.

This analysis was strengthened by the performance of correlation analysis for Eurasian sites as distant as eastern Taymyr, Siberia and northeast Sakha, Russia, both at the northern treeline. The close correlation of results from these sites imply a very widespread occurrence of the MWP.

Relation to human history

It is clear from historical records that the Medieval Warming Period could be evidenced by events in travel and exploration during the period AD 900 to 1300. For example, the Vikings had raided the shores of the British Isles through much of the period earlier than AD 900, but between AD 900 and AD 1200 their explorations intensified to the north due to more ice free seas and milder weather. That period denoted a clear trend of travel, exploration and colonisation of Greenland and Iceland.[3]

Schimel et al. argue in Evolution of the human-environment relation that the climate of the MWP was rather similar to that of 2010, and that, correspondingly, such a mild climate was an intrinsic element to ending the Dark Ages and promoting increased agricultural yields, greater trade, an increased human population and strengthened nation states. Furthermore, the Little Ice Age that followed was accompanied by widespread crop failure, plagues and othe hardships for humankind.

In North America archaeologists often term the MWP as the Medieval Climatic Anomaly, as a time interval that is not only associated with greater warmth, but also higher aridity.[4] This time interval from approximately AD 900 to 1300 is often associated with certain marginally habitable sites falling into disuse during the period, as evidenced by archaeological layers of habitation clearly dating to eras before and after the MWP;[5] this dating is conducted by both carbon dating techniques as well as identification of tools by technology era.

Clark et al observed that the severity of MWP droughts in the western USA were not dissimilar to pronounced megadroughts of the twentieth century in terms of shortfall of annual rainfall; however, the MWP megadroughts were distinguished by their extreme multi-decadal lengths.

Relation to longer term climatic cycles

The Medieval Warming Period has been placed in a larger context of climatic oscillations during the Holocene, and even over longer intervals. One of the bases of these temperature cycles has been expressed in the chemical and dust content of major Siberian air currents as measured by ice cores in the Greenland Ice Sheet Project Two.[6] These ice core records reveal a periodicity measuring approximately 2100 years, the last cooling event commencing approximately AD 1300 as attested by deposition of particulates and sea salt chemicals carried by these longitudinally driven wind currents The concentration of sea salt and particulate matter can be viewed as a proxy for enhanced lateral atmospheric circulation, particularly related to Siberian gross wind patterns.